Re: [PATCH] perf: Add irq and exception return branch types

[James Clark <james.clark@arm.com>]

On 24/02/2022 05:36, Anshuman Khandual wrote:
> This expands generic branch type classification by adding two more entries
> there in i.e irq and exception return. Also updates the x86 implementation
> to process X86_BR_IRET and X86_BR_IRQ records as appropriate. This changes
> branch types reported to user space on x86 platform but it should not be a
> problem. The possible scenarios and impacts are enumerated here.
> 
> --------------------------------------------------------------------------
> | kernel | perf tool |                     Impact                        |
> --------------------------------------------------------------------------
> |   old  |    old    |  Works as before                                  |
> --------------------------------------------------------------------------
> |   old  |    new    |  PERF_BR_UNKNOWN is processed                     |
> --------------------------------------------------------------------------
> |   new  |    old    |  PERF_BR_ERET/IRQ are blocked via old PERF_BR_MAX |
> --------------------------------------------------------------------------
> |   new  |    new    |  PERF_BR_ERET/IRQ are recognized                  |
> --------------------------------------------------------------------------
> 
> When PERF_BR_ERET/IRQ are blocked via old PERF_BR_MAX (new kernel with old
> perf tool) the user space might throw up an warning complaining about some
> unrecognized branch types being reported, but it is expected. PERF_BR_ERET
> and PERF_BR_IRQ branch types will be used for BRBE implementation on arm64
> platform.
> 
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
> Cc: Jiri Olsa <jolsa@redhat.com>
> Cc: Namhyung Kim <namhyung@kernel.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Will Deacon <will@kernel.org>
> Cc: linux-arm-kernel@lists.infradead.org
> Cc: linux-perf-users@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> ---
> This applies on v5.17-rc5
> 
> These two new branch types expands generic branch type classification but
> still leaves another three entries in 'type' field for later. Please refer
> a previous discussion [1] for some further context.
> 
> [1] https://lore.kernel.org/all/1643348653-24367-1-git-send-email-anshuman.khandual@arm.com/
> 
>  arch/x86/events/intel/lbr.c           | 4 ++--
>  include/uapi/linux/perf_event.h       | 2 ++
>  tools/include/uapi/linux/perf_event.h | 2 ++
>  tools/perf/util/branch.c              | 4 +++-
>  4 files changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/events/intel/lbr.c b/arch/x86/events/intel/lbr.c
> index 669c2be14784..fe1742c4ca49 100644
> --- a/arch/x86/events/intel/lbr.c
> +++ b/arch/x86/events/intel/lbr.c
> @@ -1329,10 +1329,10 @@ static int branch_map[X86_BR_TYPE_MAP_MAX] = {
>  	PERF_BR_SYSCALL,	/* X86_BR_SYSCALL */
>  	PERF_BR_SYSRET,		/* X86_BR_SYSRET */
>  	PERF_BR_UNKNOWN,	/* X86_BR_INT */
> -	PERF_BR_UNKNOWN,	/* X86_BR_IRET */
> +	PERF_BR_ERET,		/* X86_BR_IRET */
>  	PERF_BR_COND,		/* X86_BR_JCC */
>  	PERF_BR_UNCOND,		/* X86_BR_JMP */
> -	PERF_BR_UNKNOWN,	/* X86_BR_IRQ */
> +	PERF_BR_IRQ,		/* X86_BR_IRQ */
Hi Anshuman,

I couldn't verify if these changes to the lbr map matched up to the spec because
I couldn't find the right section. I suppose there might need to be a comment
about why BR_ERET == BR_IRET or what those abbreviations are.

I think it could also be possible to leave these entries as unknowns if we don't know
if they're even being used. It will always be possible to go back and update these
lbr mappings after you've added the new types.


>  	PERF_BR_IND_CALL,	/* X86_BR_IND_CALL */
>  	PERF_BR_UNKNOWN,	/* X86_BR_ABORT */
>  	PERF_BR_UNKNOWN,	/* X86_BR_IN_TX */
> diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
> index 82858b697c05..d37629dbad72 100644
> --- a/include/uapi/linux/perf_event.h
> +++ b/include/uapi/linux/perf_event.h
> @@ -251,6 +251,8 @@ enum {
>  	PERF_BR_SYSRET		= 8,	/* syscall return */
>  	PERF_BR_COND_CALL	= 9,	/* conditional function call */
>  	PERF_BR_COND_RET	= 10,	/* conditional function return */
> +	PERF_BR_ERET		= 11,	/* exception return */
> +	PERF_BR_IRQ		= 12,	/* irq */
>  	PERF_BR_MAX,>  };
>  
> diff --git a/tools/include/uapi/linux/perf_event.h b/tools/include/uapi/linux/perf_event.h
> index 82858b697c05..d37629dbad72 100644
> --- a/tools/include/uapi/linux/perf_event.h
> +++ b/tools/include/uapi/linux/perf_event.h
> @@ -251,6 +251,8 @@ enum {
>  	PERF_BR_SYSRET		= 8,	/* syscall return */
>  	PERF_BR_COND_CALL	= 9,	/* conditional function call */
>  	PERF_BR_COND_RET	= 10,	/* conditional function return */
> +	PERF_BR_ERET		= 11,	/* exception return */
> +	PERF_BR_IRQ		= 12,	/* irq */
>  	PERF_BR_MAX,
>  };
>  
> diff --git a/tools/perf/util/branch.c b/tools/perf/util/branch.c
> index 2285b1eb3128..a9a909db8cc7 100644
> --- a/tools/perf/util/branch.c
> +++ b/tools/perf/util/branch.c
> @@ -49,7 +49,9 @@ const char *branch_type_name(int type)
>  		"SYSCALL",
>  		"SYSRET",
>  		"COND_CALL",
> -		"COND_RET"
> +		"COND_RET",
> +		"ERET",
> +		"IRQ"
>  	};
>
Otherwise the new entries look good to me, so without the lbr changes:

Reviewed-by: James Clark <james.clark@arm.com>

If we're keeping the lbr mapping changes, then I will defer to someone else
to review.

Thanks
James

>  	if (type >= 0 && type < PERF_BR_MAX)